My gem knowledge is incredibly basic. I'd love to have gone on a Gemology course but I joined in this wonderful trade far far too late to consider doing a gemstone course as well as learning the entire making proccess from the begining.

I however, have got access to a digital refractometer, this helps massively but I got totally stumped the other day, as the batteries ran out and I had no replacements, a customer turned up looking to sell a ring and I had no way of knowing what the "Stone" was.

One was a Black cabooshon oval stone and the other a flat Black signet ring set stone.

My guess was they were Onyx, so based on this I made the guy an offer, explaining honestly my inexperiance and he was happy with the price I offered.

He did tell me, he couldn't be sure if the stone was Plastic,Glass or Onyx....So without a refractometer, is there a way to tell these apart easily ?

Thank you